An optical system is an optical device which transmits and refracts light, converging or diverging the beam. Lenses are typically made of glass, crystal or transparent plastics. A simple optical system consists of a single optical element, whereas a compound optical system, or optical assembly, is an array of two or more lenses. Thus, an optical assembly can be a doublet, triplet, or other combination of individual optical elements. In a compound optical system, two or more lenses of different shapes, which can be made of materials with different refractive indices, are arranged one after the other. Optical systems do not form perfect images, however, and there is always some degree of aberration introduced by the optical system which causes the image to be an imperfect replica of the object.
In an imaging system, aberration occurs when light from one point of an object does not converge into a single point after transmission through the optical system collinearly. Chromatic aberration is when a refractive optical system fails to focus different colors transmitted through it to a common convergence imaging point as when the material used to create lenses has a different refractive index for different wavelengths of light. There are several different basic types of aberration which can affect image quality, including astigmatism, coma, spherical aberration, distortion, etc.
An optical system with astigmatism is one where rays that propagate in two perpendicular planes going through the entrance pupil have different foci. If an optical system with astigmatism is used to form an image of a cross, for example, the vertical and horizontal lines will be in sharp focus at two different distances. Coma, or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfections of the lens or other components in an optical system. Specifically, coma is a variation in magnification over the entrance pupil. In refractive optical systems, especially those imaging a wide spectral range, coma and astigmatism are a function of field angles and to a smaller part a function of wavelength.
Spherical aberration, astigmatism and coma can be extremely difficult to compensate. Traditionally, a sophisticated series of refractive optical elements with varying properties have been introduced to correct and/or minimize aberrations. However, correcting aberrations by introducing various numbers of optical elements to the optical train increases the complexity of the system. Such an approach would be undesirable for compact configurations, and would substantially increase both the size and manufacturing cost of the system, resulting in many disadvantages, such as greater weight, package volume, fabrication and alignment costs.